1. Field of the Invention
The disclosures herein generally relate to an exposure device, which causes a light-emitting element to emit light according to a gray level of image data, and an image forming apparatus.
2. Description of the Related Art
Electro-photographic printers, which are often called laser printers, are widely used in companies and in homes, and for commercial printing. A laser printer includes a photoreceptor drum as an exposure unit, around the periphery of which a charge unit, an exposure device, a development unit and a transfer unit are arranged adjacent to each other. By selectively exposing, by the exposure device, the periphery of the photoreceptor drum, which is charged by the charge unit, an electrostatic latent image is formed. The latent image is developed with toner provided from the development unit. Then, the toner image, developed with the toner, is transferred to a paper or on to a transfer belt, by the transfer unit.
As the exposing method in the laser printer, a method of reflecting a laser light from a laser diode onto a polygon mirror, and exposing the periphery of the drum to the laser light by one line in a main scanning direction, and another method of exposing the periphery of the drum to the light of an LED (Light Emitting Diode) array of one line, arranged in the main scanning direction, by one line, are known. The latter method is called the LED array type. According to the recent developments of the characteristics of the LED and the technology in manufacturing the LED, the exposure devices of the LED array type have been actively developed. However, in the LED array type exposure device, the LEDs are arranged in a staggered arrangement in two columns. In such an exposure device, thousands of emitting points are required to be arranged with high accuracy. There is a problem that reducing the production cost of the exposure device is difficult.
Recently, an image forming apparatus, in which an organic EL (Electro Luminescence) emitting element is employed for the light emitting element in the exposure device, has been proposed. The organic EL light emitting element has a feature that the emitting point can be fabricated with high accuracy. However, luminance of the organic EL light emitting element is quite low compared with the LED. Accordingly, a light quantity from the exposure device, including the organic EL light emitting element, is insufficient for the exposure in the laser printer.
In order to solve the above problem, Japanese Patent No. 4552601 and Japanese Published Patent Application No. 2006-187895 disclose a multiple exposure by organic EL light emitting elements.
FIG. 10A is a diagram schematically showing the exposure device disclosed in Japanese Patent No. 4552601. In the exposure device 211 shown in FIG. 10A, ten lines, in the sub scanning direction (Y axis direction), of light emitting elements are formed. Between the even-numbered line and the odd-numbered line, the organic EL light emitting elements 203 are arranged in a staggered manner. That is, the exposure device shown in FIG. 10A includes five groups, each of which has the organic EL light emitting elements arranged in a staggered manner. The organic EL light emitting elements in respective groups and on the same column can expose the same drawing point on the photoreceptor drum. Accordingly, the drawing point on the photoreceptor drum can be exposed five times at the maximum.
FIG. 10B is a diagram schematically showing the exposure device disclosed in Japanese Published Patent Application No. 2006-187895. In this exposure device 311, for example, the arrays of the organic EL light emitting elements 303A and 303B, are formed so that in each line of the arrays an area of a light emitting pixel for each of the organic EL light emitting elements is the same. The areas of the light emitting pixel for the organic EL light emitting element are different from each other between the lines of the arrays. The area is an integer multiple of the smallest area. Accordingly, the exposure device disclosed in the Japanese Published Patent Application No. 2006-187895 can perform sixteen levels of exposure, including the minimum level in which none of the organic EL light emitting elements are selected (do not emit light).
However, in the exposure device disclosed in Japanese Patent No. 4552601, the light quantities from the organic light emitting element are the same, and the expression for N gray levels requires (N−1) lines, in the sub scanning direction, of arrays of organic EL light emitting elements. Furthermore, the number of the shift registers, which perform delay processing for data in the sub scanning direction, in the sub scanning direction is the same as the number of the light emitting elements.
In the exposure device disclosed in Japanese Published Patent Application No. 2006-187895, an expression of larger number of gray levels is possible with a lesser number of organic EL light emitting elements. However, in this exposure device, the resolution is constrained by the largest area of the organic EL light emitting element, and there is a problem that improvement of the resolution is difficult. Furthermore, when the pitch among the organic EL light emitting elements is uniform, all the organic EL light emitting elements are arranged with the pitch for the largest area of the organic EL light emitting elements. Accordingly, there is a problem that the size of the exposure device becomes large.